Here is a quick guide on assigning dual momentary footswitch options to your HX Stomp or HX Stomp XL using the external switch jack:
Global Settings > Preferences > Page 2 will show EXP FS Tip and EXP FS Ring. These setting should be assigned to FS4/FS7 and FS5/FS8 accordingly.
Next menu item over:
Global Settings > Footswitches > FS4(FS7) Function, FS5(FS8) Function assignment on page 2. These can be assigned to different functions such as Stomp buttons OR Preset up and down.